Drill Bit Metamorphism: Recognition and Impact on Show Evaluation

Abstract With petroleum exploration and development progressing into deeper and more challenging environments, drilling technology has provided the means to effectively and safely reach and complete desired targets. Approaches such as turbo-drilling with polycrystalline-diamond compact (PDC) bits or diamond-impregnated bits increase penetration rates, lessen drill-bit changes, and are more easily steered in directional drilling, but also generate artifacts that may hinder recognition of indigenous hydrocarbons and interfere with geochemical interpretations. The higher bit speeds with turbo-drilling compared to rotary drilling (e.g., ~1000 vs. 300 rpm, respectively) often generate additional heat that may essentially fuse rock cuttings with drilling mud, thermally crack oil-based drilling fluids, and produce both hydrocarbon and non-hydrocarbon artifacts. Bit-generated rock textures and pulverized drill cuttings, mixed with drilling mud cause difficulty in evaluation of lithology, age, and shows. The recognition of indigenous vs. artifact products is crucial to determining the presence and characteristics of hydrocarbons encountered in the well bore. These can be distinguished through analyses of mud gas and fluid-inclusion volatiles (FIV) analyses of drill cuttings. Analyses of mud gases show that gases typically not found in natural gases are generated during drill-bit metamorphism (DBM) including CO, ethene, and propene. Gases are rich in methane and C2-C4+ hydrocarbon gas components and may include H2S, CO2 and carbonyl sulfide (COS). Laboratory experiments suggest that thermal cracking of oil-based drilling mud can be the major contributor to these gases. Cracking of base oil alone generated wet gas with little CO2 and no H2S, while cracking of base oil in drilling mud generated wet gas with abundant CO2 and H2S. Drilling mud additive components such as lignosulfonate are likely sources of the generated CO2 and H2S. Recognition of DBM is crucial to effective drilling operations and evaluation of indigenous hydrocarbons. Introduction As petroleum exploration, development and production proceeds to greater water and well-penetration depths and more challenging environments there are increasing pressures on drilling organizations to quickly and safely reach and complete the objective reservoir targets. Unfortunately, the drill bits and bit-drive mechanisms that allow increased penetration rates often further complicate geological and geochemical interpretations by altering drill cuttings returns and generating artifact products. High temperatures and strain-rates experienced at the drill-bit/rock interface can generate very high temperatures that can essentially fuse drilled rock with drilling mud, which are then quickly quenched away from the bit resulting in highly altered cuttings returns to the surface. Thus potentially very useful geologic information is obscured, including lithologic, paleontologic and geochemical properties. Cuttings are often the major source of geologic information on which important operational decisions are based. High bit temperatures can also result in the generation of drilling artifacts, such as both hydrocarbon and non-hydrocarbon gases, that complicate recognition of shows while drilling.